ASTRONOMERS PLAN CLOSE-UP LOOK AT HALLEY

Jon Van, Science writer. Chicago TribuneCHICAGO TRIBUNE

Astronomers plan to capitalize on public interest in the return of Halley`s Comet later this year to launch their most intensive study ever of comets, which may hold important clues to the formation of the solar system.

The International Halley Watch has recruited more than 900 professional astronomers in 47 countries and more than 400 advanced amateurs in 37 countries to provide observational data on the comet, making it the most ambitious cooperative scientific endeavor ever undertaken, said Ray Newburn Jr., leader of the group in this hemisphere.

Newburn, an astronomer with NASA`s Jet Propulsion Laboratory, said there are several reasons for such an extensive effort devoted to Halley`s Comet.

''Halley is important because it is the brightest comet whose returns can be accurately predicted,'' Newburn told a meeting of the American Association for the Advancement of Science here last week.

''And, to be honest, it is the only comet whose `charisma` is such as to make large amounts of telescope time available at the world`s largest observatories. Hence the International Halley Watch.''

Halley`s Comet sweeps through the inner solar system and past Earth in its long orbit around the sun once every 76 years. It is named for Edmond Halley, the British astronomer who predicted in 1705 that the comet would return in 1758-59 (''Halley,'' according to researchers, is pronounced to rhyme with ''valley'').

The comet named for Halley had been observed as far back as 240 B.C. by Chinese astronomers, but until Halley`s work, it was thought that each sighting represented a different comet rather than recurrent visits by a single stellar body completing an elliptical orbit that takes it beyond the orbital path of the planet Neptune.

Despite man`s long awareness of comets, real knowledge of them is remarkably scarce.

Comets and asteroids, two of the smaller constituents of the solar system, have been largely ignored by spacecraft exploration of Earth`s neighbors, David Morrison, a University of Hawaii astronomer, told the meeting here.

''It`s like bypassing two or three planets, and it`s time we made up for the omission,'' Morrison said.

Halley will make close approaches to Earth Nov. 27, when it will be 58 million miles away; and the following April 11, when it will be 39 million miles away.

During March, 1986, five international spacecraft, two from Japan, two from the Soviet Union and one from the European Space Agency, will make brief flyby encounters with the comet. Experiments aboard NASA`s space shuttles and on airplanes, balloons and sounding rockets are also planned. Two U.S. spacecraft, Pioneer 12 orbiting Venus and the Solar Maximum Mission satellite orbiting Earth, will train instruments on Halley.

Newburn noted that observations from the International Halley Watch will be essential to aid engineers in directing the sending of spacecraft to the right position for encounter with the comet, which is estimated to have a nucleus some four miles across.

The nucleus is thought to be a ''dirty snowball'' made up of frozen gases, water and rocky material. For most of its orbit, the comet is in the deepest of freezes, but it thaws during the period of its orbit nearest the sun, causing it to give off molecules from its outer layer, producing the bright halo of dust and gas surrounding the nucleus called a coma.

Observers have noted that Halley`s Comet has already begun developing its coma, and they aren`t sure why it is doing this while still quite far from the sun. Early observations have also revealed that the comet`s brightness is fluctuating, again to the puzzlement of scientists.

Morrison noted that most information gleaned from the Halley visit will concern the gases producing its coma and tails and their reaction with solar wind and other energies near the sun.

Within a decade, scientists hope to position a spacecraft in orbit for months to fly alongside a comet and learn more about the nucleus itself.

Before the turn of the century, Morrison said, they hope to land on a comet`s nucleus, retrieve some material and return it to Earth for extensive study.

It is expected that cometary nuclei are among the purest in the solar system, having changed less than any other matter since the solar system was formed some 4.6 billion years ago, said Laurel Wilkening of the University of Arizona.

Despite all the excitement among researchers, no one expects that Halley`s visit will make much impact upon the general public.

Because of the geometric configuration between Halley`s Comet, the sun and Earth, its visit will be one of the least bright sights on record, Newburn said. It will be quite low in the skies as seen from the Northern Hemisphere. People will need to get away from the lights of large cities such as Chicago to get a view.

''You`ll be able to see it,'' Newburn said, ''but it won`t knock your eyes out.''